noob question, how do I do it? I did it once by walking into one but any other time I either trade places or accidentally defeat them :( can't really find anything in the game about the controls for it either but I could be wrong

ok nvm I figured it out.
Once a girl is helpless you gotta click the shackles on the bottom right and then click her, so instead of attacking her it'll capture her. Not super intuitive (in my opinion)

It should capture her if you run into her without passthrough enabled, assuming she has any binding she will stay at 0hp and disappear into collection when captured. Im not sure what you mean by shackles on the bottom right?